To pinpoint the context of this invention, it may be recalled that it is sometimes preferred to sample charges rather than voltages, so as to reduce the influence of the clock noise (sometimes also called clock jitter) when one wishes to sample a high-frequency signal under the control of a clock which defines the periodic-sampling phases. By integrating not a voltage level in a sampling capacitor but a current for a known sampling duration, the influence of this clock noise is reduced. But, as the input signal to be converted generally takes the fowl of a voltage (or more exactly of high-frequency voltage variations), it is necessary to place upstream of the sampling capacitor or capacitors a high-quality transconductance amplifier which will convert the voltage variations very precisely into current variations.
Sample-and-hold units using a transconductance amplifier at the input stage are employed notably in applications of sampling telecommunications signals at high frequency with a view to a frequency transposition followed by an analog-digital conversion of the signals. The frequency bands of telecommunications signals are very congested. A telecommunications channel uses a narrow frequency band and everything arising outside of this band constitutes nuisance noise. Generally, the telecommunications signal sent on a given channel comprises a useful spectrum centered on a carrier frequency Fo. The carrier with frequency Fo is amplitude modulated or frequency modulated and one of the aims of the sampling is in particular to transpose the signal to an intermediate-frequency spectrum (centered on an intermediate frequency Fi which is lower than Fo) or even to a baseband spectrum (centered on a zero frequency).
Now it is known that the sampling, at a sampling frequency Fe, of a signal whose useful spectrum is centered on a frequency Fo produces on the one hand a useful spectrum in a transposed band centered on an intermediate frequency Fi=Fe−Fo, but also produces in this transposed band what is called noise spectrum aliasings; this aliased noise originates not only from the noise present in the useful band centered on Fo, but also from the noise present in bands of the same width centered on Fi+Fe, or indeed on other bands still related to the harmonics of Fe, such as a band centered on KFe+Fi or KFe−Fi, K being an integer. All these varieties of noise are aliased towards the new useful signal band centered on the intermediate frequency Fi, and the resulting noise is the sum of the aliased noise since the noise arising from the various bands is uncorrelated and remains uncorrelated after aliasing.
If a filter were placed at the input of the sample-and-hold unit, this would eliminate part of the noise, but this would not eliminate the inherent noise of the input amplifier of the sampler. If filtering is done at the output, it is too late, the aliased noise is already in the useful output passband.
In practice, it would be necessary to filter actually inside the input amplifier of the sampler. Unfortunately, most input amplifier structures do not allow such filtering internal to the amplifier without degrading the performance of the amplifier.
By way of example, the article by B. Nauta “A CMOS transconductance-C Filter Technique for Very High Frequencies” in IEEE Journal of Solid-State Circuits, vol 27 No. 2 pp 142-153 February 1992 describes a transconductance amplifier structure which has good passband characteristics but correlatively high noise and it does not easily enable filtering of this noise.
Another example is given in the article by M. Koyama et al., “A 2.5-V Active Low-Pass Filter Using All-n-p-n Gilbert Cells with a 1-Vp-p Linear Input Range” in IEEE Journal of Solid State Circuits, vol. 28, no. 12, pp 1246-1253, December 1993. This involves a two-stage amplifier which can support internal filtering, but which exhibits distortion.
In still another example, T. Kwan and K. Martin., “An adaptive analog continuous-time CMOS biquadratic filter”, in IEEE Journal of Solid State Circuits, vol. 26, no. 6, pp 859-867, June 1991, the introduction of an internal filter would disturb the useful signals.
It has been found that it was possible to modify the structure of the output stage of a transconductance amplifier intended to be placed at the input of a sample-and-hold unit, so as to introduce thereinto an internal filtering fulfilling in the best possible way a function for eliminating the noise which is engendered by aliasing during the sampling performed downstream.
The output stage is constituted according to the invention as a folded cascode stage absorbing the whole of the current produced by the transconductance amplifier and it is to this folded cascode stage that a filter is linked which is capable of diverting the current variations which are not in the useful frequency band of the input signal to be sampled, without diverting the current variations which are in the useful band. The filter has a high impedance in the useful band centered around the frequency Fo of the carrier.